Practice it.
arms up and jump and while you jump, tuck at the same time
Tuck Jump, Pike Jump, Straddle Jump, Swivle Hips, Barani, Somersault... There are just a few of many.
pike, straddle, split, tuck, layout, hollow, arch, and open or closed hip position
Tuck Jump, Pike Jump, Straddle Jump, Swivle Hips, Barani, Somersault... There are just a few of many.
Jump and tuck your head and exhale through your nose and just keeping practicing!! 
A tu is when a gymnast folds her knees into her chest. This can be done in a jump or a flip.
Jumping jack, tuck jump, squat thrust, push ups, sit ups, sprint.
handstand, forward roll, tuck jump dismount, side handstand dismount
There are many different skills! They range from a tuck jump to a back tuck. In all levels you compete floor,vault,beam, and bars but they are all different skills according to level.
A back tuck is a tumbling move. In a back tuck, the body is curled up tightly into a ball. The knees are as close to the chest as possible, with the hands clutching the knees. By "tucking" together tightly in this manner, the body is able to reach maximum angular velocity, thus minimizing the time required for the body to complete its revolution.
You dont, what you do is ride it and tuck up your bike under you. Because you want to go fast, but you dont wanna go high.